Mercedes CLK

	This layout is smaller and lighter than a hybrid transmission

When it comes to boosting efficiency in petrol engines, Mercedes is playing catch-up. Premium rival Lexus already has hybrid cars, while BMW offers regenerative braking to improve fuel economy on several models.

But the three-pointed star is working hard to regain ground – and Auto Express has driven its latest development: a CLK 500 with ISG technology.

The initials stand for Integrated Starter/Generator, and in lay terms, the set-up is a stop-start system. A disc-shaped electric motor is added between the engine and automatic transmission, while a secondary battery is charged by regenerative braking and offers a power boost during hard acceleration. The main advantage is that this layout is smaller and lighter than a hybrid transmission.

For the driver, it means fuel economy is up 10 per cent from 24.8mpg to 27.3mpg, while CO2 output is esti­mated to drop by a similar proportion.
The stop-start technology cuts the engine when you come to a halt, and fires it up as soon as your right boot comes off the brake. By the time you are pressing the accelerator, it’s as if the V8 has been running all the time.
The powerplant also switches off if you shift the auto box into Park mode.

At least that’s how it’s supposed to work. But in the 30-degree heat of our urban drive in Germany, it didn’t. Despite having several opportunities to cut out as we stopped at traffic lights and junctions, it failed every time.

We thought the system wasn’t working properly, but later learned
it was because we had turned the CLK’s air-conditioning on. Apparently, the car prioritises driver comfort over saving the planet. On a second run with the cooling system switched off, the ISG technology worked fine.

Arguably, the Mercedes stop/start isn’t as clever as it thinks it is. Yes, it can cut the engine to save fuel, but similar devices offered by rival manufacturers switch off when they realise you are coming to a halt. That means owners get every last mile from the fuel tank by freewheeling to a standstill. What’s more, bosses won’t offer the ISG technology until 2009.

Rival: BMW 3-Series
impressively, BMW has managed to get its stop-start set-up on road cars already, as part of the Efficient Dynamics package – but it’s limited to four-cylinder engines for now.